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std::ranges:: viewable_range

From cppreference.net
C++
Ranges library
Range adaptors
Defined in header <ranges>
template < class T >

concept viewable_range =
ranges:: range < T > &&
( ( ranges:: view < std:: remove_cvref_t < T >> &&
std:: constructible_from < std:: remove_cvref_t < T > , T > ) ||
( ! ranges:: view < std:: remove_cvref_t < T >> &&
( std:: is_lvalue_reference_v < T > ||

( std:: movable < std:: remove_reference_t < T >> && ! /*is-initializer-list*/ < T > ) ) ) ) ;
(since C++20)

The viewable_range concept is a refinement of range that describes a range that can be converted into a view through views::all .

The constant /*is-initializer-list*/ < T > is true if and only if std:: remove_cvref_t < T > is a specialization of std::initializer_list .

Example

Run this code 
#include <ranges>
#include <string>
#include <vector>
struct valid_result {};
struct invalid_result {};
template <typename T>
concept valid_viewable_range = std::same_as<T, valid_result>;
template <typename T>
concept invalid_viewable_range = std::same_as<T, invalid_result>;
auto test_viewable_range(std::ranges::viewable_range auto &&) -> valid_result;
auto test_viewable_range(auto&&) -> invalid_result;
int main()
{
    auto il = {1, 2, 3};
    int arr []{1, 2, 3};
    std::vector vec{1, 2, 3};
    std::ranges::ref_view r{arr};
    std::ranges::owning_view o{std::string("Hello")};
    static_assert(requires {
        { test_viewable_range(il) } -> valid_viewable_range;
        { test_viewable_range(std::move(il)) } -> invalid_viewable_range;
        { test_viewable_range(arr) } -> valid_viewable_range;
        { test_viewable_range(std::move(arr)) } -> invalid_viewable_range;
        { test_viewable_range(vec) } -> valid_viewable_range;
        { test_viewable_range(std::move(vec)) } -> valid_viewable_range;
        { test_viewable_range(r) } -> valid_viewable_range;
        { test_viewable_range(std::move(r)) } -> valid_viewable_range;
        { test_viewable_range(o) } -> invalid_viewable_range;
        { test_viewable_range(std::move(o)) } -> valid_viewable_range;
        { test_viewable_range(std::ranges::ref_view(o)) } -> valid_viewable_range;
    });
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3481 C++20 viewable_range accepted an lvalue of a move-only view rejects
P2415R2 C++20 viewable_range only accepted non- view rvalues that are borrowed_range accepts more types

See also

(C++20)
a view that includes all elements of a range
(alias template) (range adaptor object)